1. Field of the Invention
This invention relates to a continuous coking process for production of cokes suitable for the production of carbon and graphite shapes by pyrolysis of a hydrocarbon mixture containing aromatic compounds.
2. Description of the Prior Art
A number of coking or pyrolysis processes adapted to the particular starting material used including petroleum tars, coal tar pitches, pyrolysis tars and other hydrocarbon mixtures containing relatively large fractions of polynuclear aromatic compounds are available for the production of cokes. Examples of these coking processes are the retort or chamber coking process used for the production of pitch cokes, fluidised bed coking processes and the delayed coking process which is used for the production of high quality cokes. In the last of these processes, selected hydrocarbon mixtures are heated to around 500.degree. C. in a tubular heater. Premature coking of the starting material in the heater is prevented by subjecting the starting material to turbulent flow and high rates of travel or velocity through the tubular heater or by adjusting temperature and pressure profiles in the heater. To convert the higher aromatic fractions into green coke, the preheated starting material is then introduced into coke drums which are operated under pressures of up to about 10 bars and at overhead temperatures of from 450.degree. to 480.degree. C. After a certain filling level has been reached in one coke drum, say two-thirds-three-fourths or more, the supply of preheated material to that drum is interrupted and the preheated material is introduced into another coke drum during whose filling the green coke is removed from the first drum. When the second drum is filled with coke to the proper level, the supply of preheated material is then switched back to the first drum and so on. The residence time or coking time of the green coke in the coke drums thus fluctuates considerably, for example between 24 hours at the base of the drum where the preheated material first settles in the empty drum and one hour at the head of the drum when the coke is almost up to the maximum level. The quality of the green coke varies considerably as a function of the residence time; for example the amount of volatile constituents therein may amount to less than 5% at the bottom of the drum and to more than 20% at the head or top of the drum. Since only some of these differences are eliminated by the subsequent heating or calcining of the green coke at a temperature of from about 1100.degree. to 1400.degree. C., most of the calcined cokes also show variations in important quality parameters, particularly in porosity, pore size distribution, particle strength and coefficient of thermal expansion. If cokes of this type are used for example for the production of carbon and graphite shapes, the products obtained will generally not be of uniform quality and will not satisfy all the demands made of them on account of the variations in the quality of the starting coke.
Further disadvantages of the delayed coking process include the undesirably high cost of the twin coke drums and the poor energy utilization factor of the process. In general, the green coke is removed hydraulically from the coke drums, size-reduced, graded and subsequently delivered to a calcining apparatus which takes the form, for example, of a rotating cylindrical furnace or disc furnace. The heat content of the coke is lost without being utilized.
The retort or chamber coking process is likewise characterized by poor utilization of energy, in addition to which cokes produced by this process are not suitable for the manufacture of high quality graphite products for the same reasons as coke produced by the fluidized bed process. The inadequate properties of the cokes for this purpose are primarily attributable to the structure of the coke particles which contain only a small fraction of acicular elements.